Shanghai’s skyline glitters with sleek financial skyscrapers and glossy residential towers, but below the city’s lustrous facade lies an enduring problem. Thanks to mass urban migration, soft soil and global warming, Shanghai is sinking, and has been for decades. Since 1921, China’s most populous city has descended more than 6 ft. Across China, land subsidence affects more than 50 cities, where 49,000 sq. mi. of land have dropped at least 8 in.

It’s not just the numbers that are frightening: the problem has manifested itself tragically and more frequently of late. Earlier this month, a young woman unexpectedly fell through the sidewalk into a 20-ft.-deep sinkhole while walking along the street in Xi’an. In April, a woman died after falling through the sidewalk into a pit of boiling water in Beijing. Scientists have continuously warned of dire repercussions if the government does not implement more stringent guidelines for urban planning, water usage and carbon emissions — and they expect the situation to get much worse in areas with large-scale, fast-paced construction, like Shanghai.

As progress continues on Asia’s soon-to-be-tallest skyscraper, the Shanghai Tower, the problem has manifested itself in malicious cracks nearby, captured and posted by users of the Twitteresque microblogging site Sina Weibo, then published by ChinaSmack. In mid-February, one blogger posted about a 22-ft.-long crack situated near the 101-story Shanghai World Financial Center, across the street from the highly anticipated tower.

In response to bloggers’ concerns, Shanghai Tower Construction, the company responsible for building the tower, issued a statement saying surface cracks were “controlled and safe,” the Shanghaiist reported. Liu Dongwei, chief architect of the China Institute of Building Standard Design & Research, cited groundwater, rainfall and soft soil foundation as the reasons for the settlements. But that’s only partially accurate.

Shanghai has inherently soft soil because of its geographical position at the mouth of the Yangtze River basin and, yes, groundwater accounts for nearly 70% of land subsidence; however, experts say, the weight of skyscrapers and global warming also play hefty roles in aggravating what they call “the most important geological disaster in Shanghai.” Unfortunately, the implications will only grow graver with the pace of development and rising sea levels.

According to a report by the Shanghai Geological Research Institute, the physical weight of skyscrapers accounts for 30% of Shanghai’s surface subsidence. “Usually groundwater pumping is the key factor,” Jimmy Jiao, a professor of earth sciences at Hong Kong University, tells TIME. “But in Shanghai, development is also important because the building density is high, and most of the high-rise buildings are sitting on the areas with soft soil.” Basically, what’s happening is, the weight of high-rise towers presses down on the earth, as if you were to put a weight on a spring or scale.

The most densely packed city in China, Shanghai reached a population of 23 million in 2010, according to census data. With the soaring number of denizens flocking to the seaside city, developers have stacked skyscrapers and thousands of high-rises side by side like dominoes. The country as a whole built 200 new skyscrapers in 2011, and by 2016, the total number is expected to exceed 800.

“As the saying goes, the more you build, the more they come,” Jiang Li, a professor of civil engineering at Baltimore’s Morgan State University who grew up in Tianjin, China, tells TIME. Pretty soon there will be 30 million people in Shanghai, while Beijing is just short of 20 million people. “At this point, so much construction has already been done that the areas are hopeless,” he says. Today’s problems have been aggravated by decades of overdevelopment and overpumping of groundwater resources.

The problems began in the 19th century, when Shanghai transformed into a trading port and began attracting both foreigners and relocating Chinese migrants. By 1900, the population had tripled to more than 1 million. People started consuming more groundwater than the overlying turf could handle, and the problem worsened dramatically. By the 1950s and early ’60s, the area started sinking 4 in. per year. The pace slowed after 1963, when the government banned a significant number of wells. To take further precaution, the government also began pumping water back into underground reservoirs. Every day, Shanghai is redirecting 60,000 tons of water through 121 wells, China Daily reported. Even with these restrictions in place, the city has descended 16 in. in the last 50 years.

Shanghai may have had this problem before the 1950s, but it didn’t start emerging in other cities until the early ’80s. Now more than 50 cities across the country face sinking problems, according to a report by the China Geology Survey. Three regions in particular have “serious land-subsidence problems,” including the Yangtze River delta area, the Fenhe River–Weihe River basin and the North China Plain. According to CCTV, Cangzhou, a city in north China’s Hebei province, has descended nearly 7 ft. In 2009, the city had to demolish a three-story building housing a branch of the city’s People’s Hospital because the first level sank so low that it fell underground.

Though some critics argue that the Chinese government has been too slow to act, research, public concern and some hefty bills ($35 billion in Shanghai alone in the past 40 years) have sparked some momentum. Recently the state council approved China’s Land Subsidence Prevention Project, a countrywide initiative to prevent land subsidence. Likewise, Beijing, which has descended more than a foot in the past decade, has also made an effort to reduce underground-water extraction, with plans to close 800 water-extraction wells this year, according to the Beijing Water Authority. By 2014, the city hopes to halt underground-water extraction in urban areas altogether as part of the North-South Water Diversion Project. The project expects to deliver 3 billion cu. ft. of water supply to Beijing from the Yangtze River. This would not only satisfy one-third of the city’s total water demand but would also cut the extraction of underground water in half.

But Li, who worked at the Chinese Academy of Science for 15 years, says such programs will not be enough. “It’s hard to quantify how much this might help, but the question is, Is that a problem solved? The answer is no. The problem lies in the early issue with urbanization,” he says. Scientists expect the regulations to help curb the consumption of underground-water supplies, but there are a few things the government has less control over, like global warming. As land degradation and excessive guzzling of groundwater continue, environmentalists predict waters surrounding Shanghai will rise 9 to 27 in. by 2050 as a result of melting ice caps.

“If you look at Shanghai during high tide, you can see the water level is higher than the streets but separated by the wall,” Li says. “This is a situation where if you have a major disaster like a hurricane, tsunami or tropical storm, it can cause serious damage.” He is especially worried about severe flooding in the coastal areas, where the majority of Chinese migrants have settled. The only way to really solve the problem is to reduce — or better yet, stop — groundwater pumping. Another option is to decrease the density of buildings, which would mean fewer heavy skyscrapers, perhaps an unrealistic solution for China’s rapidly growing cities.

In the meantime, Li suggests local governments impose water restrictions and fees to encourage less wasteful consumption of water. He also proposes more secondary water uses, where wastewater is recycled for washing cars or watering plants. Even then, global warming remains an obstacle. As skyscrapers in Shanghai go up and the glaciers in the North and South poles melt down, cities like Shanghai grow more and more vulnerable every day.